Ecosystem Response To 15 Years Of Chronic Nitrogen Additions At The Harvard Forest LTER, Massachusetts, USA

  • Sites: US-Ha2, US-LPH
  • Publication Type: JOUR
  • Authors: Magill, A. H.; Aber, J. D.; Currie, W. S.; Nadelhoffer, K. J.; Martin, M. E.; McDowell, W. H.; Melillo, J. M.; Steudler, P.

  • Humans have altered the global and regional cycles of nitrogen more than any other element. Alteration of N cycling patterns and processes in forests is one potentially negative outcome of accelerated N deposition worldwide. To assess potential impacts of N deposition on temperate forests, a series of chronic nitrogen additions in two contrasting forest types (red pine plantation and mixed hardwood stand) were designed as a core experiment of the Harvard Forest (HF) Long-term Ecological Research (LTER) program. This paper describes the chronic N experimental study site in detail and presents the long-term baseline measurements established at the beginning of treatments in 1988.

    Results reported here continue or accelerate trends presented in previous papers. Losses of inorganic N remain high in the high N plots (higher in pines than hardwoods) and low N plots in the pine stand also have measurable DIN losses. Foliar and fine root N concentrations are elevated significantly. Mortality of red pine reached 56% by 2002 in the pine high N plot, and biomass accumulation has stopped altogether. The high N hardwood stand shows increased ANPP, but excess N availability and a severe drought in 1995 contributed to mortality of 72% of red maple trees by 2002. Species importance and litterfall patterns were altered in several plots after 1995. Roots, foliage and wood have diminished as net sinks for added N, re-emphasizing the role of soils in N retention. Two mechanisms for large net retention of added N were suggested in a review paper in 1998. Of these, abiotic immobilization is supported by a growing set of papers, while assimilation and re-exudation by mycorrhizae is suggested by increased DON concentrations.

  • Journal: Forest Ecology And Management
  • Funding Agency: —
  • Citation Information:
  • Volume: 196
  • No: 1
  • Pages: 7-28
  • Publication Year: 2004/07
  • DOI: 10.1016/j.foreco.2004.03.033